This invention relates to a powder core for use in a choke coil and, in particular, to a powder core excellent in d.c. superposition characteristic and frequency characteristic.
For a choke coil used at a high frequency, a ferrite core or a powder core is used. In these cores, the ferrite core is disadvantageous in that the saturation flux density is small. On the other hand, the powder core produced by forming metal powder has a high saturation flux density as compared with soft magnetic ferrite and is therefore advantageous in that the d.c. superposition characteristic is excellent.
However, since the powder core is produced by mixing the metal powder and an organic binder or the like and compaction-forming the mixture under a high pressure, insulation between powder particles can not be kept so that the frequency characteristic of the permeability is degraded. In case where the binder is mixed in a large amount in order to assure the insulation between the powder particles, a space factor of the metal powder is reduced so that the permeability is decreased.
In recent years, energy saving and global warming due to carbon dioxide are growing into serious problems. In view of the above, energy saving strategy is rapidly developed in domestic electrical appliances and industrial apparatuses. To this end, it is required to increase the efficiency of an electric circuit. As one of solutions, it is strongly desired to improve the permeability of the powder core, the frequency characteristic, and the core loss characteristic.
In an existing method of improving the permeability of the powder core, a principal point is put on an improvement of a packing fraction of magnetic powder. For this purpose, it is proposed, for example, to increase a forming pressure. If the packing fraction is improved in this manner, however, the insulation between the powder particles is degraded to result in an increase in eddy current loss and deterioration in frequency characteristic.
It is therefore an object of this invention to solve the above-mentioned problem and to provide a powder core excellent in d.c. superposition characteristic and in frequency characteristic.
In order to solve the above-mentioned problem, a study has been made of a method of interposing an insulator between magnetic particles in a powder core. As a result, this invention has been made. As a result of progress in studying how to embody the above-mentioned method, the present inventors found out that the insulator can be interposed between the magnetic powder particles by mixing a raw material of the powder core with powder or a solution containing an SiO2-producing compound and MgCO3 or MgO powder, and pressing and heat-treating a resultant mixture.
According to one aspect of this invention, there is provided a powder core obtained by compaction-forming magnetic powder, wherein the magnetic powder is an alloy comprising 1-10 wt % Si, 0.1-1.0 wt % O, and balance Fe, an insulator comprising SiO2 and MgO as main components being interposed between magnetic powder particles having a particle size of 150 xcexcm or less.
According to another aspect of this invention, there is provided a high-frequency reactor comprising the above-mentioned powder core and a winding wound around the powder core.
According to still another aspect of this invention, there is provided a method of producing the above-mentioned powder core, comprising the steps of mixing magnetic powder, at least one of silicone resin and a silane coupling agent, and at least one of MgCO3 powder and MgO powder, compaction-forming a resultant mixture into a compact body, and heat-treating the compact body thus obtained.
This invention provides the powder core excellent in d.c. superposition characteristic and frequency characteristic as compared with an existing powder core using the similar magnetic powder. It is understood that, by heat treating the mixture of the SiO2-producing compound and MgCO3 or MgO powder, a glass layer comprising SiO2 and MgO as main components is formed between magnetic particles so that insulation between the particles can be assured without decreasing a packing fraction.